The present invention relates to dynamoelectric machines and, more particularly, to dynamoelectric machines of the type employing carbon brushes making sliding contact with a metallic contact member.
Many types of dynamoelectric machines employ carbon brushes contacting a segmented metallic commutator for transferring electric power between a stationary and a rotating member. The best-known of such dynamoelectric machines include DC motors and generators, and AC induction motors of the type having wired rotors. In all of these devices, rotor windings are individually connected to metallic contact members, such as commutator bars or collector rings, arranged to form a cylindrical surface with which the bearing surfaces of stationary carbon brushes make sliding contact.
One skilled in the art will recognize that the metallic contact members may also be annular slip rings contacted by stationary brushes. For compactness of description, the following disclosure is directed toward segmented commutators. It should be understood that the invention applies equally to apparatus having slip rings.
Although carbon brushes contacting metallic commutator bars or collector rings have been known since the very early days of rotating electrical machinery, the exact manner in which the electricity is transferred is not completely understood.
In a conventional commutator-brush system, the brushes are intended to wear at a controlled rate which is slow enough to avoid excessive machine down-time for their replacement. The metallic commutator is intended to be relatively durable preferably with a lifetime measured in years. It has been found that several conditions can arise which drastically increase the wear rate of the brushes and the commutator.
In a properly operating commutator-brush system, a lubricating conductive oxide film is formed on the surface of the commutator running in the presence of atmospheric moisture and oxygen. The conductive oxide film maintains the contact resistance between the brush face and the commutator base metal at a satisfactorily low value. The lubricating qualities of the conductive oxide film reduce brush and commutator wear. The brushes and commutator are maintained in contact with the atmosphere in order to provide access to oxygen and moisture.
It is believed that moisture lubricates relative slippage of the interfaces between the carbon planes thereby to permit slippage for conforming the carbon to the metal interface, and thus to reduce abrasion. In the complete absence of moisture, the carbon from which carbon brushes are made is so abrasive that the lifetimes of brushes and commutators are very short.
If the amount of moisture is insufficient, abrasion increases substantially. Even when sufficient moisture is present, high brush temperatures can prevent penetration of the moisture into the carbon brushes. For example, it has been observed that brush temperatures exceeding about 100 degrees C. lead to rapid wear.
Wear is accelerated when a large number of brushes are connected in parallel for carrying a high current. If the contact resistance of one brush rises higher than that of its neighbors, current is selectively carried by the brushes having lower contact resistance. This process leads to rapid brush wear which may proceed in an avalanche fashion with rapid brush destruction.
Excessive oxidation of the brushes is a principal cause of rapid brush wear. The presence of atmospheric contaminants, acidic or alkaline materials, oil vapors and materials capable of catalyzing the oxide film or the brush carbon, may produce such excessive oxidation and lead to excessive brush and/or commutator wear.
Some such atmospheric contaminants, such as solid particles, may be filtered out of the air entering the region containing the brushes and commutator. Filtering is less successful against chemical vapors which are especially capable of promoting excessive oxidation. However, even when filtering is successful, the necessity to supply air and moisture to the conventional commutating process essentially precludes sealing the brush-commutator system in a sealed enclosure.
As noted, one important attribute of the brush-commutator interface is lubrication. While a certain amount of lubricant such as, for example, drying oils, is conventionally formed within carbon brushes, it is insufficient to provide all of the necessary lubrication.
In order to seal against all external contaminants without requiring moisture control, some means must be found to provide the required additional lubrication. External oils and greases are not favored on the commutator surface since, to be effective, these require a lubricating film which is so thick that it produces a high contact resistance with all of the consequences of selectivity and catastrophic rapid brush wear noted above. Other types of lubricants such as, for example, molybdenum disulfide, although satisfactory in carefully controlled vacuum applications, can be broken down by an electrical arc into abrasive compounds such as, for example, molybdenum tri-oxide.
No satisfactory lubricating technique has heretofore been disclosed which permits satisfactory operation of a carbon-brush/contact-member system while avoiding the above causes of excessive brush and/or commutator wear.